1. Technical Field
Illustrative embodiments described in this patent specification generally relate to a rotary drive device that drives a rotary body and an image forming apparatus including the rotary drive device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
There are many rotary bodies used in the image forming apparatus. Examples of the rotary bodies include, but are not limited to, the photoconductor, a drive roller that drives belt members such as an intermediate transfer belt and a transfer belt included in the transfer device, and a conveyance roller that conveys the sheet or the like.
A change in rotary speed of the photoconductor or the intermediate transfer belt for example, causes jitter or uneven image density in a resultant image. Consequently, continuous speed variation in the photoconductor or the intermediate transfer belt at a certain frequency periodically causes uneven image density throughout the resultant image, resulting in stripes, or banding. In addition, speed variation in the photoconductor shifts a sub-scanning position of an exposure line from a writing system and a sub-scanning position upon primary transfer of a toner image from the photoconductor onto the intermediate transfer belt. Further, speed variation in the intermediate transfer belt shifts a sub-scanning position upon secondary transfer of the toner image from the intermediate transfer belt onto the sheet as well as upon primary transfer of the toner image. Thus, speed variation in the photoconductor and the intermediate transfer belt considerably degrades image quality.
Therefore, steady, consistent driving of these bodies is important for good imaging, and accordingly, there is a longstanding need for a mechanism that transmits torque with less rotational fluctuation from a drive source to a target rotary body to be driven to meet the requirement for highly accurate driving of the rotary body. This requirement may be met by the materials used for the constituent rotary parts.
Further, in order to achieve sufficient durability for plastic gears, use of a planetary gear mechanism has been proposed. The planetary gear mechanism includes a sun gear rotated by torque from a drive source, an annular gear having inward-facing teeth and provided coaxially to the sun gear, multiple planetary gears provided within the annular gear at equal intervals in a circumferential direction of the annular gear to respectively engage the sun gear and the annular gear, and a carrier rotatable coaxially to the sun gear and the annular gear to rotatably support the planetary gears. The torque from the drive source rotates the sun gear so that the multiple planetary gears are rotated around their own axes. At the same time, the multiple planetary gears revolve around the sun gear within the annular gear to rotate the carrier. Torque generated by rotation of the carrier is transmitted to the rotary body via an output shaft that couples the carrier to the rotary body. Thus, use of the multiple planetary gears in the planetary gear mechanism diversifies rotational loads, thereby achieving the necessary durability.
Plastic gears manufactured by injection molding of molten resin have been used as drive transmission members that transmit torque from the drive source to the photoconductor or the intermediate transfer belt, each of which is required to be accurately driven. The plastic gears are superior to metal gears due to their higher self-lubricating property, lower noise during operation, lighter weight, superior corrosion resistance, and easier mass producibility. At the same time, however, plastic gears are inferior to the metal gears in terms of lower durability, lower dimensional accuracy, and lower rigidity.
In particular, the mold used for manufacturing the annular gear with injection molding includes upper and lower parts and a cavity formed between the upper and lower parts. The cavity is a cylindrical space having teeth shapes along an inner, circumference thereof. The molten resin is injected into the cavity, and is cooled and solidified to form the annular gear. The teeth shapes of the cavity are transferred onto the molten resin when solidified to form the inward-facing teeth of the resultant annular gear. Multiple pin gates through which the molten resin is injected into the cavity are provided to the upper part of the mold at equal intervals in a circumferential direction of the cavity. Accordingly, the molten resin can be evenly injected into the cavity from each of the pin gates.
However, because the molten resin injected from the pin gates into the cavity spreads radially around each of the pin gates, the farther the positions within the cavity are away from each of the pin gates, the longer it takes for the molten resin to reach them. Consequently, the amount of molten resin tends to be lacking around intermediate positions between each of the two adjacent pin gates, that is, the farthest positions from each of the pin gates, possibly causing deformation in the inward-facing teeth of the resultant annular gear. Such deformation in the teeth of the annular gear causes rotary transmission errors due to irregular engagement of the planetary gears and the annular gear, resulting in rotary speed fluctuation in the planetary gear mechanism. As a result, it is difficult to transmit the torque with less rotational fluctuation from the drive source to a target rotary body to be driven.